1. Field of the Invention
The present invention relates in general to the field of signal processing, and more specifically to a system and method for providing overload prevention for delta-sigma modulators using a multi-order, feedback loop filter topology.
2. Desription of the Related Art
Delta sigma modulators produce a high resolution output signal using a low-resolution quantizer. Delta sigma modulators achieve the high resolution by using oversampling data rates higher than the Nyquist rate. Many signal processing systems include delta sigma modulators to quantize an input signal into one or more bits at a high rate to produce a high resolution output signal. “Delta-sigma modulators” are also commonly referred to using other interchangeable terms such as “sigma-delta modulators”, “delta-sigma converters”, “sigma delta converters”, and “noise shapers”.
FIG. 1 depicts a basic N-order delta sigma modulator 100. The delta sigma modulator 100 includes a difference stage 101 to determine a difference signal ud(n) between an input signal x(n) and a quantizer feedback signal y(n−1). The digital input signal x(n) represents an oversampled version of an input signal from a signal source 103, such as a microphone or audio/visual playback device. The input signal can be any type of input signal, such as an audio input signal. A preprocessor generally includes a digital interpolator to generate the oversampled digital input signal. The quantizer feedback signal y(n−1) represents a one (1) cycle delay of the quantizer output signal y(n). Delta sigma modulator 100 includes a loop filter 102 to filter the difference signal ud(n). The loop filter 102 is implemented using a feed-forward type loop filter topology of order N, wherein N is a positive integer. The loop filter 102 includes N series connected filter stages 104.0, 104.1, . . . , 104.N−1. Each of the filter stages 104.0, 104.1, . . . , 104.N−1 includes an integrator, whose z domain representation is depicted in the exploded schematic 106. The output signals of all filter stages 104.0, 104.1, . . . , 104.N−1 are adjusted by respective coefficients c0, c1, . . . , cN−1 (also referred to as “weight factors”) and added together by an adder stage 110 to generate a filter output/quantizer input signal uf(n). Each integrator of the filter stages 104.0, 104.1, . . . , 104.N−1 also generally includes a gain coefficient (not shown) that is determined using well-known digital design techniques to achieve the desired frequency response of the loop filter 102. For low frequency baseband signals, such as audio signals (approx. 0 Hz to 25 kHz), the loop filter 102 functions as a low pass filter during non-quantizer overload conditions with a relatively high gain for frequencies in the baseband.
The quantizer 108 provides a single-bit or multi-bit output signal y(n). The quantizer introduces a quantization error. Filtering of the quantization error by the delta sigma modulator 100 minimizes the quantization error at low, baseband frequencies. Thus, the delta sigma modulator 100 exhibits “noise shaping” behavior by reducing noise in the baseband. Subsequent signal processing of output signal y(n) can filter out signal frequencies, including noise, located above the baseband.
The attainable signal-to-noise ratio (SNR) of the delta sigma modulator 100 in the baseband depends in part on the amount of oversampling of the original input signal and the order of the loop filter 102. Higher order filters require less oversampling to maintain acceptable SNR. However, the order is limited by instabilities in the loop filter 102. Instabilities in the loop filter 102 can be caused by, for example, large sample-to-sample excursions of the input signal x(n). The order of loop filter 102 is also limited by overload of the quantizer 108. Quantizer overload occurs when the quantizer input signal uf(n) exceeds an input signal peak operating range of the quantizer 108. In multi-order delta sigma modulators, estimating a maximum range of input signal x(n) that will with certainty prevent the quantizer input signal ud(n) from a obtaining a value outside the operation range of quantizer 108 is difficult to determine exactly. Estimations of a maximum range for input signal x(n) remains difficult because of the complexity of causal relationships, the influence of noise generated by the delta sigma modulator from non-ideal physical components, rounding errors, and/or other factors.
Consequently, one solution to prevent quantizer overload conservatively limits the amplitude range of the input signal x(n) to values that with a high degree of certainty prevent the delta sigma modulator 100 from going into quantizer overload. However, conservatively limiting the range of input signal x(n) also artificially limits the dynamic range of input signal x(n). A second solution for preventing quantizer overload is to clip the quantizer input signal uf(n) and, thus, limit the quantizer input signal uf(n) to values within the non-overload operating range of quantizer 108. However, conventional techniques that limit the quantizer input signal uf(n) suffer from poor signal-to-noise ratios at least during clipping operations.
U.S. Pat. No. 5,243,345 (referred to herein as the “Naus Patent”) describes an embodiment of the second quantizer overload prevention solution. The Naus Patent describes a delta sigma modulator that includes a feed-forward type loop filter and signal limiters to regressively limit the quantizer input signal uf(n) while providing less restrictions on the amplitude range for the input signal x(n). The Naus Patent is entitled “Sigma-Delta Modulator Having a Plural Order Loop Filter with Successive Filter stages of Successively Smaller Signal Excursion Range”, inventors Naus et al., filed Feb. 21, 1992, and assigned to U.S. Philips Corp.
FIG. 2 depicts one embodiment of the delta sigma modulator 200 described by the Naus Patent. The delta sigma modulator 200 includes an N-order, low pass loop filter 202 having series connected filter stages 204.0, 204.1, . . . , 204.N−1. Each filter stage 204.x includes an integrator 206.x and a limiter 208.x to limit the output signal amplitude of each filter stage 204.x, where xε{0, 1, . . . , N−1}. The output signals of all filter stages 204.0, 204.1, . . . , 204.N−1 are adjusted by respective coefficients c.0, c.1, . . . , c.N−1 (also referred to as “weight factors”) and added together by an adder stage 208 to generate a filter output/quantizer input signal uf(n). The loop filter 202 functions in accordance with the transfer function:uf(n)/ud(n)=G0G1. . . GN−1cN−1+ . . . +G0G1c1+G0c0,
where G0G1, . . . GN−1 represent the gains of respective filter stages 204.0, 204.1, . . . , 204.N−1. The limiters 210.0, 210.1, . . . , 210.N−1 regressively limit the filter stage output signals to respective limit values L0, L1, . . . , LN−1.
The limiters prevent instabilities in the delta sigma modulator 200 in the event of an increasing input signal 210.0, 210.1, . . . , 210.N−1 by limiting the filter output signal uf(n) with the last limiter 210.N−1 and systematically regressively limiting the output of immediately preceding filter stages. The systematic, regressive limitation of preceding filter stages is accomplished by setting limiting values Lx in accordance with:
                              L          x                                      G            0                    ⁢                      G                          1              ⁢                                                                            ⁢          …          ⁢                                          ⁢                      G            x                              <                        L                      x            -            1                                                G            0                    ⁢                      G                          1              ⁢                                                                            ⁢          …          ⁢                                          ⁢                      G                          x              -              1                                            ,                  ⁢    where        x    ∈                  (                              0            ,            1                    ,          …          ⁢                                          ,                      N            -            1                          }            .      
However, the noise performance of the Naus Patent delta sigma modulator suffers during overload conditions because the signal transfer function (STF) of the feed-forward loop filter peaks in the near out of band frequencies, thus, severely compromising low pass performance and accentuating noise during overload.
Researchers continue to seek solutions to prevent quantizer overload, achieve faster quantizer overload recovery times, and maintain acceptable signal-to-noise ratios.